1. Field of the Invention
The invention relates to a variable gain amplifier and more particularly to an input stage of an amplification line of a reception circuit, which for example is of a type used in transmission systems.
The amplification lines of reception circuits require a set of characteristics that are difficult to reconcile with one another: -- the gain, which is variable, should be high, the input impedance should have a low value and should be independent of the gain, the noise factor should be low, and the compression point should be high.
2. Description of the Prior Art
In the prior art, there are known ways of making the input stages of such amplification lines by the parallel connection of several amplifiers of the common emitter type that are switched according to the gain desired. It is then necessary to add a low-value resistor in parallel at the input of the circuit so that the input impedance has a low value and is independent of the gain.
The performance characteristics in terms of noise are then much lowered.
The invention does not have this drawback.